In golf balls having a core enclosed in a cover, ionic ethylene copolymers known as ionomer resins have been widely used as the cover material because of their high rebound, impact resistance, cut resistance and ball flying performance. Use of ionomer resins as the cover material is the mainstream in the current golf ball manufacture.
Ionomer resins are generally available in the form of ethylene-metal unsaturated carboxylate-unsaturated carboxylic acid copolymers and such copolymers further containing an unsaturated carboxylic acid ester. Well-known ionomer resins are those wherein the metal salts of unsaturated carboxylic acids are salts of monobasic unsaturated carboxylic acids such as acrylic acid and methacrylic acid and dibasic unsaturated carboxylic acids such as maleic acid and fumaric acid with monovalent metals such as sodium and potassium and divalent metals such aszinc and magnesium. The ionomer resins most often used as the cover material of golf balls are those wherein the metals are sodium and zinc, that is, sodium ion neutralized type, zinc ion neutralized type and a mixture thereof.
Although golf balls using such ionomer resins as the cover have excellent characteristics as mentioned above, they still have some room to improve their durability against hitting. In accordance with the recent tendency of golf balls toward softer feeling, especially the widespread use of golf balls having a soft core, there is a need for a golf ball having the soft core enclosed in a cover having good hitting durability that few cracks occur after repeated shots.
An object of the invention is to provide a golf ball meeting such a requirement.